Of the various electrostatic printing techniques, the most familiar and widely utilized is that of xerography wherein latent electrostatic images formed on a charge retentive surface, such as a roller, are developed by a suitable toner material to render the images visible, the images being subsequently transferred to plain paper. This process is called an indirect or transfer processes because it first forms a visible image on an intermediate photoreceptor and then transfers that image to a paper surface.
Another form of electrostatic printing is one that has come to be known as direct electrostatic printing (DEP). This form of printing differs from the aforementioned xerographic form in that pigmented particles (i.e. toner or developing material) are deposited directly onto a plain (i.e. not specially treated) information carrier to form a visible image. In general, this type of printing device uses electrostatic fields controlled by addressable electrodes for allowing passage of pigment particles through selected apertures in a printhead structure. A separate electrostatic field is provided to attract the pigment particles to an imaging substrate in image configuration.
Many of the methods used in field imaging, (e.g., creating an electric field pattern in the print zone), such as particle charging, particle transport, and particle fusing are similar to those used in "laser printers". However, the novel feature of direct printing is its simplicity of simultaneous field imaging and particle transport to produce a visible image directly on plain paper.
U.S. Pat. No. 3,689,935 granted to Pressman discloses a method to produce text and pictures with pigment particles on an information carrier, directly from computer generated signals, without the need for those signals to be intermediately converted to another form of energy such as light energy, as is required in electrographic printers like laser printers.
Pressman discloses an electrostatic line printer incorporating a multilayered particle modulator or printhead comprising a layer of insulating material, a continuous layer of conducting material on one side of the insulating layer and a segmented layer of conducting material on the other side of the insulating layer. At least one row of apertures is formed through the multilayered particle modulator. Each segment of the segmented layer of the conductive material is formed around a portion of an aperture and is insulated from every other segment of the segmented conductive layer. Selected potentials are applied to each of the segments of the segmented conductive layer while a fixed potential is applied to the continuous conductive layer. An overall applied field projects charged particles from a particle source through the row of apertures. The density of the particle stream is modulated according to the pattern of potentials applied to the segments of the segmented conductive layer. The modulated stream of charged particles impinge upon a print-receiving medium interposed in the modulated particle stream and translated relative to the particle modulator to provide line-by-line scan printing.
A drawback to the Pressman device is that the particle source must be an airborne stream of charged particles. That stream of airborne particles is of low particle density, resulting in very poor contrast on the print-receiving medium. In addition, it is very difficult to effectively control the airborne particle stream.
U.S. Pat. No. 5,036,341 granted to Larson discloses a solution where control of many adjacent wire pairs is more effective in attracting toner particles from a magnetic toner carrier, increasing the density of toner particles deposited on the print-receiving medium and providing a more effective means of controlling particle transport. The Larson '341 patent discloses a method which begins with a stream of electronic signals defining the image information. A uniform electric field is created between a high potential on the back electrode and a low (0 volt) potential on the developer sleeve. That uniform field pattern is modified by potentials on selectable wires in a two-dimensional wire mesh array placed in the print zone. The wire mesh array consists of parallel control wires, each of which is connected to an individual voltage source, across the width of the paper surface. The multiple wire electrodes, called print electrodes, are aligned in adjacent pairs parallel to the motion of paper; the orthogonal wires called transverse electrodes are aligned perpendicular to the paper motion. All wires are initially at a V.sub.w (white) potential, preventing all toner transport from the developer sleeve. As image locations on the paper surface pass beneath wire intersections, adjacent transverse and print wire pairs are set to a V.sub.b (black) potential to produce an electrostatic field drawing the toner particles from the developer sleeve. The toner particles are pulled through the apertures being formed in the square region between four crossed wires (i.e. two adjacent rows and two adjacent columns), and deposited on a paper surface in the desired visible image pattern. The toner particle image is then made permanent by heat and pressure fusing the toner particles to the surface of the paper.
In Larson '341, one voltage source can affect a plurality of apertures, reducing the number of circuits needed for the printer. For example, in a device with M rows and N columns, the number of electronic drive circuits is reduced from M*N to M+N. This power sharing technique is termed multiplexing. However, a drawback in the device of the Larson '341 patent is that during operation of the control electrode matrix, the individual wires can be sensitive to the opening or closing of adjacent apertures, resulting in undesired printing due to the thin wire border between apertures. This defect is call cross-coupling.
International Patent Application PCT/SE90/00398, also by Larson, discloses a method to substantially reduce cross-coupling defects in the wire lattice electrode matrix by using an array of looped wires to enclose the apertures for the passage of pigment particles. Using two wires connected as a loop for each dot position results in more effective control of the adjacent wire pairs since only one electronic drive circuit is connected to each electrode loop across the linear array. The looped wire electrodes, arranged in a two-dimensional matrix of rows and columns, can be constructed as a woven wire mesh or a laminated structure using etched circuit fabrication methods. The etched circuit fabrication methods are preferred for reasons of accuracy, repeatability, and automated assembly. The woven wire mesh alternates the row and column electrode distance within each aperture so that the electrode matrix acts as if all of the electrodes are substantial at a uniform distance from the particle carrier.
However, a two-layer etched control electrode circuit does not perform well because the layer closest to the particle carrier dominates in controlling the opening and closing of apertures. The control electric fields acting between the control electrode matrix and the particle carrier are very sensitive to the distance between the control electrode matrix surface to the particle carrier surface. If the rows and columns are at different distances, as with layered circuit boards, their ability to accurately control the electric fields is greatly reduced. A single layer control electrode matrix would be more effective in controlling the apertures.
U.S. Pat. No. 5,121,144 granted to Larson shows a control electrode matrix on a single insulating layer with one circular electrode surrounding each passage to eliminate the cross-coupling. The electrodes are arranged in rows and columns on a single insulating substrate with a single electronic drive needed for each electrode. The ring electrode design requires a single electronic driver for each dot position and is effective in eliminating cross-coupling and increasing maximum print speed, but increases the complexity and manufacturing costs of the device by an undesirable amount because of the large number of electronic drivers required.
U.S. Pat. Nos. 3,689,934 and 4,814,796, and GB Patent No. 2,108,432 disclose a control electrode matrix design, each of which is also disadvantaged by the requirement of one electronic drive circuit for each aperture surrounded by an individual electrode.
Thus, there is a need for a device to control a two-dimensional array of control electrodes located on a single layer insulating substrate to reduce cross-coupling and reduce manufacturing cost.